In the art of making paper with modern high-speed machines, sheet properties must be continually monitored and controlled to assure sheet quality and to minimize the amount of finished product that is rejected when there is an upset in the manufacturing process. The sheet variables that are most often measured include basis weight, moisture content, and caliper (i.e., thickness) of the sheets at various stages in the manufacturing process. These process variables are typically controlled by, for example, adjusting the feedstock supply rate at the beginning of the process, regulating the amount of steam applied to the paper near the middle of the process, or varying the nip pressure between calendaring rollers at the end of the process. Papermaking devices well known in the art are described, for example, in "Handbook for Pulp & Paper Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., and "Pulp and Paper Manufacture" Vol. III (Papermaking and Paperboard Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 4,982,334, 4,786,817, and 4,767,935.
In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (wet stock) on a traveling mesh papermaking fabric and water drains by gravity and vacuum suction through the fabric. The web is then transferred to the pressing section where more water is removed by dry felt and pressure. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is essentially a de-watering system. In the sheetmaking art, the term machine direction (MD) refers to the direction that the sheet material travels during the manufacturing process, while the term cross direction (CD) refers to the direction across the width of the sheet which is perpendicular to the machine direction.
A wide range of chemicals is utilized in the papermaking stock furnish to impart or enhance specific sheet properties or to serve other necessary purposes Such additives as alum, sizing agents, mineral fillers, starches and dyes are commonly used. Chemicals for control purposes such as drainage aids, defoamers, retention aids, pitch dispersants, slimicides, and corrosion inhibitors are added as required. The order of addition must be taken into account to prevent interaction at the wrong time and enhance retention in the paper sheet.
Wet end chemistry deals with all the interactions between furnish materials and the chemical/physical processes occurring at the wet end of the papermaking machine. The major interactions at the molecular and colloidal level are surface charge, flocculation, coagulation, hydrolysis, time-dependent chemical reactions and microbiological activity. These interactions are fundamental to the papermaking process. For example, to achieve effective retention, drainage, sheet formation, and sheet properties, it is necessary that the filler particles, fiber fines, size and starch be flocculated and/or adsorbed onto the large fibers with minimal flocculation between the large fibers themselves.
There are three major groups involved in wet-end chemistry: solids, colloids and solubles. Most attention is focused on the solids and their retention. In order to maximize retention, it is important to cause the fines and fillers to approach each other and form bonds or aggregates which are stable to the shear forces encountered in the paper machine headbox and approach system. In modern papermaking, this is usually accomplished by using synthetic polymers.
Control of wet-end chemistry is vital to ensure that a uniform paper product is manufactured. If the system is allowed to get out of balance (e.g., by over-use of cationic polymers), the fibers themselves will become flocculated and sheet formation will suffer. Also, functional additives (e.g., sizes, wet-strength agents) are often added at the wet end; if the chemistry is not under control, the functionality may not be adequately imparted and the product will be off-quality.
As is apparent, there is a wide range of phenomena which can influence the fundamental interactions at the molecular and colloidal. One of these factors is the electrokinetics. In this regard, the term, zeta potential, applies to the electrical charges existing in fine dispersions. Referring to FIG. 6, a solid particle (e.g., fiber, starch, mineral) suspended in a papermaking stock is surrounded by a dense layer of ions having a specific electrical charge. This layer is surrounded by another layer, more diffuse than the first, that has an electrical charge of its own. The bulk of the suspended liquid also has its own electrical charge. The difference in electrical charge between the dense layer of ions surrounding the particle and the bulk of the suspended liquid is the zeta potential, usually measured in millivolts. The zeta potential, .zeta., and is defined by the equation: ##EQU1## where q is the charge on the particle, .delta., is the thickness of the zone of influence of the charge on the particle, and D is the dielectric constant of the liquid. Measurements of zeta potential can give an indication of the effectiveness of added electrolytes in lowering the energy barrier between colloids, and thus can serve to guide the selection of optimum conditions for coagulation.
The best retention of fine particles and colloids in the papermaking system normally occurs when the zeta potential is near zero. Pulp fibers, filler and size particles usually carry a negative charge, but the zeta potential can be controlled by absorbing positive ions from solution. Polyvalent cations such as aluminum and ferric are most effective.
Papermakers alum, Al.sub.2 (SO.sub.4).sub.3, is still a commonly used agent for wet end chemistry because it effectively neutralizes the negatively-charged fiber and pigment particles to zero zeta potential. At the proper pH, it also hydrolyzes to form an ionic polymer that has a significant flocculating effect by bridging from particle to particle and thereby forming large ionically-attracted flocs.